Supported cobalt catalysts wherein the cobalt is in its elemental or reduced state are well known and find use in many reactions involving hydrogen such as hydrogenation reactions, e.g. nitrile hydrogenation reactions and the Fischer-Tropsch synthesis of hydrocarbons. The activity of the catalysts is believed to be directly proportional to the cobalt surface area of the reduced catalysts, but in order to achieve high cobalt surface areas, the cobalt should be well dispersed on the support. As the cobalt content of a catalyst increases above 15%, particularly above 20% by weight (on reduced catalyst) the cobalt becomes more difficult to disperse resulting in lower cobalt surface areas per gram cobalt. Cobalt is a relatively expensive metal and therefore there is a desire to improve the cobalt dispersion. (expressed as cobalt surface area per gram cobalt) for supported cobalt catalysts.
Cobalt catalysts comprising cobalt on zinc oxide/aluminium compositions are known. EP-B1-0671976 describes cobalt/zinc oxide catalysts wherein the zinc oxide contains a group IIIa metal such as aluminium at 0.5-7.5% preferably 0.8-2% by weight (as metal on weight of zinc oxide). The low levels of aluminium, present in the composition in the form of a spinel (i.e. ZnAl2O4), were apparently included to increase the compression strength or reduce shrinkage and sintering. Whereas the cobalt content of the Co/Zn/Al catalysts is given as 3-40% wt (Co metal on total weight of composition), only 10% wt was exemplified and no indication of the cobalt surface area was given. Furthermore, the catalyst preparation method exemplified required firstly co-precipitating the zinc oxide composition, filtering, washing, drying and calcining it, then slurrying the calcined composition with cobalt nitrate solution, drying the slurry, grinding and re-calcining. The multiple heating steps in this method make it unattractive for large scale catalyst manufacture. Consequently a method whereby a Co/Zn/Al catalyst is formed in fewer steps, in particular without the need for two-calcination steps is desirable.
EP-A-1358934 describes particulate cobalt-zinc co-precipitated catalysts having a volume average particle size of less than 150 μm and their use for the Fischer-Tropsch synthesis of hydrocarbons. Whereas it is stated that a group IIIa element such as aluminium, may be present in a concentration of 0.1-10% wt (on catalyst) to effect structural stability, no specific disclosure of aluminium-containing catalysts is made, nor is any data provided on the cobalt surface areas of the resulting catalysts.